To make parts suitable for demanding friction applications such as aircraft braking, high temperature materials such as carbon-carbon composites, carbon and ceramic fiber reinforced preforms, and carbon and ceramic foams are densified by Chemical Vapor Deposition/Chemical Vapor Infiltration (CVD/CVI) and/or by liquid infiltration with a resin or with pitch. Densification is accomplished by converting the resin or pitch within the preform into carbon.
Impregnation of porous bodies with resins and pitches typically involves vacuum/pressure infiltration (VPI). In the VPI process, a volume of resin or pitch is melted in one vessel while a porous preform is contained in a second vessel under vacuum. The molten resin or pitch is transferred into the porous preform contained in the second vessel using a combination of vacuum and pressure. The VPI process is limited to using resins and pitches that possess low viscosity and associated low carbon yields, so that several impregnation cycles are ordinarily required to achieve the desired final density.
The carbon yield of pitches can be enhanced by high pressure impregnation/carbonization processes. However, high pressure vessels are capital intensive and of limited size, thereby limiting the number of performs that can be densified in a single vessel. The very high pressures used also increase the risk of explosion. Alternatively, one can use liquid resins that have high carbon yields (>80%). Typical high char yield resins include synthetic mesophase pitches (e.g., AR mesophase pitch from Mitsubishi Gas Chemical Company, Inc., a catalytically polymerized naphthalene) as well as thermally or chemically treated coal tar and petroleum derived pitches. However the high viscosity and associated high processing temperatures of these materials is problematic.
Resin Transfer Molding (RTM) technologies are widely used in the aerospace, automotive, and military industries as a means of densification of porous performs. RTM is often used for the production of polymer based composites. A fibrous preform or mat is placed into a mold matching the desired part geometry. Typically, a relatively low viscosity thermoset resin is injected at low temperatures (100-300° F., 38-149° C.), using pressure or induced under vacuum, into a porous body contained within a mold. The resin is cured within the mold and the part is then removed from the mold.
U.S. Pat. No. 4,986,943 discloses a method for oxidation stabilization of pitch-based matrices from carbon-carbon composites. In this method, a lattice work of carbon fibers is infiltrated with a pitch-based matrix precursor, oxidized in an oxygen-containing atmosphere at a temperature below the pitch softening point, and carbonized to convert the matrix material into coke.
U.S. Pat. No. 5,248,467 teaches an apparatus for use in a VPI method. A mold cavity containing fibers and/or inserts is placed under vacuum and then the molding material is injected into the cavity under vacuum. The patent teaches that injection of the matrix molding material can be from any location on the mold, because there is nothing to displace and no need to consider flow characteristics of the matrix material in terms of displacing air toward a vent.
U.S. Pat. No. 5,306,448 discloses a method form resin transfer molding which utilizes a reservoir. The reservoir comprises a pressure yield porous sponge containing from 2 to 10 times the sponge's weight in resin. The resin reservoir facilitates resin transfer molding by providing a resin reservoir that can ensure the desired impregnation of a porous preform such as a porous fiber reinforced composite.
U.S. Pat. No. 5,770,127 describes a method for making a carbon or graphite reinforced composite. A rigid carbon foam preform is placed within a sealed flexible bag. A vacuum is created within the bag. Matrix resin is introduced into the bag through an inlet valve to impregnate the preform. The preform is then cured by heating. The resulting carbon or graphite structure is then removed from the bag.
In typical resin extrusion processing, a viscous melt is forced under pressure through a shaping die in a continuous stream. The feedstock may enter the extrusion device in the molten state, but often it consists of solid particles that are subject in the extruder to melting, mixing, and pressurization. The solid feed may be in the form of pellets, powder, beads, flakes, or ground material. The components may be premixed or fed separately through one or more feed ports. Many extruders incorporate a single screw rotating in a horizontal cylindrical barrel, with an entry port mounted over one end (feed end) and a shaping die mounted at the discharge end (metering end). Twin screw extruders are widely employed for difficult compounding applications and for extruding materials having high viscosity. Twin screw designs can be either counter-rotating or co-rotating, with the screws intermeshing or not intermeshing. A series of heaters can be located along the length of the barrel. In RTM processes, the shaping die at the metering end is replaced with a mold containing a porous body or preform.
U.S. patent application Ser. No. 09/653,880, now U.S. Pat. No. 6,537,470 B1, describes tooling that enables resin infiltration of porous preforms (e.g., flat annular brake disk performs) from the top and bottom simultaneously. This tooling and melt flow pattern works well for many fiber architectures. However, low density nonwoven fabric-based preforms are often better infiltrated employing the “through thickness” infiltration of the present invention.
Thus, in some cases, infiltrating a thick porous disk from both top and bottom simultaneously creates a risk of damaging the preform, since when two melt streams meet in the interior of the web during the resin fill process, an opposing force is created. The force initiates a wedge-type effect as it drives the resin melt streams, and any gases trapped within the porosity of the preform, towards the inside diameter (ID) and outside diameter (OD) locations within the fiber matrix of the preform. With some fiber architectures, i.e., low density nonwovens, this flow in the plane is problematic, and results in delaminations, cracks, etc., at various melt injection pressures, in the preform that is being melt infiltrated with resin. Specifically, nonwoven preform precursors having low densities (<1.1 g/cc), after a first cycle of CVD, especially large diameter preforms (>16 inches), may delaminate during RTM processing using the apparatus described in application Ser. No. 09/653,880.